1. Field of the Invention
The present invention relates to an original reading device that reads an original by a sheet-through method, and in particular to technology for detecting and appropriately correcting noise origination from debris that has adhered to an original platen.
2. Description of the Related Art
Original reading devices included in copiers, facsimile devices, scanner devices, etc. often employ a sheet-through type of original reading method in which an optical system such as a reading line sensor is fixed in place, an original is conveyed by an automatic original conveying device so as to pass over the line sensor, and image data corresponding to the image of the original is generated.
This sheet-through type of original reading method has advantages such as improving reading speed over a method in which the optical system is moved while reading an image. On the other hand, if debris adheres to the original platen over the line sensor, line-shaped noise appears along the direction in which the original is scanned.
Color line sensors normally include three sensors, that is to say, one for each color, and the three colors of sensors are arranged parallel to each other at a specified pitch of roughly several pixels. Therefore, in the case of line noise appearing due to small-diameter debris, the specified pitch can be used to correct the data that corresponds to the line noise portion, based on data from a color sensor over which debris has not adhered.
Patent documents 1 to 5 are examples of conventional technology for correcting line noise.
Patent document 1 (Japanese Unexamined Patent Application No. 2000-78409) discloses an image reading device that includes two reading parts. The image reading device detects pixels that have different read values, compares each of the read values to the read values of surrounding pixels, and determines a read value that has a large difference from the read values of surrounding pixels to be noise. Regardless of whether the noise included in read values is black or white, read values that are largely different from the color in the surrounding area are determined to include noise and are eliminated, thus obtaining read values that do not include noise.
Patent document 2 (Japanese Unexamined Patent Application No. 2002-271631) discloses an image reading device which includes two reading units that have different spectral sensitivities and are offset in the sub-scanning direction of the manuscript. The image reading device compares density values resulting from the reading performed by the two reading parts, judges whether an edge component in the main scanning direction is included in one of the reading results, and detects a noise component based on the comparison result and judgment result. According to patent document 2, if the reading result includes a noise component due to the influence of a foreign object such as debris, the noise component is detected based on the comparison result and judgment result, thereby eliminating the need to include redundant reading parts that have a spectral sensitivity characteristic for reading color image information.
Patent document 3 (Japanese Unexamined Patent Application No. 2005-94685) discloses an image formation device that includes reading elements corresponding to the colors red, green, and blue, and a monochrome reading element. A full-color scanner that uses a sheet-through method can output high-quality image data when reading a monochrome image, and even if debris is present in the optical reading path, black lines can be precisely and effectively removed from monochrome image data and color image data, thus enabling high-quality image output.
Patent document 4 (Japanese Unexamined Patent Application No. 2003-8846) discloses an original reading device that exposes an original to light to read information before the original has reached a reading position, and obtaining and storing image data pertaining to one of a plurality of colors. When performing original reading, the original reading device retrieves the stored image data and performs color correction on the data obtained by the original reading, with respect to the color of the stored image data. This structure enables correcting color lines when performing full-color reading by a sheet-through method, without needing to provide the original feeding device with additional mechanisms.
Patent document 5 (Japanese Unexamined Patent Application No. 2004-297302) discloses an image reading device that detects the presence of line images for each color. The structure recited in patent document 5 enables reliably detecting line images originating from dust, dirt, etc. in a color image obtained by reading an original.
However, in the above method that makes use of the pitch, all of the colors of sensors may simultaneously be affected by debris that is larger than the pitch, or even by small-diameter debris if the pitch between the different colors of sensors has been reduced. In such cases, the line noise portion of data cannot be properly corrected.